The present invention relates to a process for treating a gas containing fluorine-containing compounds and CO, and particularly it relates to a process and an apparatus for efficiently treating an exhaust gas containing fluorine-containing compounds and CO which are discharged in the step of dry cleaning the inner surfaces and the like of a semiconductor manufacturing apparatus, the step of etching various types of formed films such as oxide films and the like in the semiconductor industry.
In the semiconductor industry, various types of harmful gases are being used in the semiconductor manufacturing steps and the environmental pollution by discharging them to the environment is a matter of concern. Particularly, in an etching step, a CVD step and the like in the semiconductor industry, fluorinated hydrocarbons such as CHF3 and fluorine-containing compounds such as perfluoro-compounds including CF4, C2F6, C3F8, C4F8, C5F8, SF6 and NF3 (hereinafter referred to as xe2x80x9cPFCxe2x80x9d) are being used. It is of urgent necessity to establish a system of removing the fluorine-containing compounds present in the exhaust gas from these steps as the global warming gases. Further, the exhaust gas from these steps sometimes contains CO as the processing gas or frequently contains CO which has been generated by exposing a mixed gas of PFC with O2 to a plasma in a chamber.
As the method of removing PFC in a gas, a gas treating method using an alumina based catalyst obtained by incorporating various metals into alumina; a gas treating method using alumina having an Na content as the metal of not greater than 0.1% by weight; a gas treating method comprising contacting a molecular oxygen with a gas in the presence of alumina; a gas treating method using an aluminum-containing catalyst in the presence of steam at a temperature of 200 to 800xc2x0 C.; a gas treating method using various types of metal catalyst in the presence of a molecular oxygen and water; and the like have been proposed. Furthermore, as described in the specification of Japanese Patent Application No. 2000-110668, a gas treating method using xcex3-alumina having a specific crystal structure (which exhibits diffraction lines having an intensity of 100 or more at the five angles of 33xc2x0xc2x11xc2x0, 37xc2x0xc2x11xc2x0, 40xc2x0xc2x11xc2x0, 46xc2x0xc2x11xc2x0 and 67xc2x0xc2x11xc2x0 of the angles of diffraction 2xcex8 measured by an X-ray diffraction apparatus) as the catalyst has been proposed.
On the other hand, as the method for removing CO in a gas, there is a method comprising oxidizing CO using a hopcalite oxidation catalyst (a composite oxide of Cu and Mn, an Ni oxide, and the like) and O2 to CO2 which is then removed. However, the technique of simultaneously treating fluorine-containing compounds and CO has not yet been reported.
Accordingly, in order to treat both fluorine-containing compounds and CO in a gas according to the conventional technique, it is necessary to adopt a method comprising forming a two-stage catalytic reaction vessel having a CO oxidation catalyst arranged in the previous stage and xcex3-alumina arranged in the later stage and passing a gas therethrough to oxidize CO in the previous stage and to decompose the fluorine-containing compounds in the latter stage. However, in this instance, there is a problem that the fluorine in the fluorine-containing compounds present in the gas comes to a catalytic poison against the CO oxidation catalyst to remarkably lower the CO oxidative power in the reaction vessel of the previous stage and as a result, the CO cannot be treated to a threshold limit value-time weighted average concentration (TLV-TWA value) of 25 ppm or smaller in a short period of time.
Furthermore, in order to treat a gas containing fluorine-containing compounds and CO according to the conventional technique, there are such problems that each component has to be treated with a different catalyst, which needs to use different heating vessels to be individually filled with different catalysts, and the treating temperature for each heating vessel has to be individually controlled, and further a wide space for installing the apparatuses is necessary, and the temperature control becomes complicated. Further, there is a problem of complicating the control of catalysts since the period of exchanging the catalyst differs due to the difference in the lives of individual catalysts. In addition, there is a problem of an increase in the running cost such as the expense necessary for periodically exchanging the catalysts.
Then, the object of the present invention is to solve the above described problems according to the conventional technique and to provide a process and an apparatus for treating a gas containing fluorine-containing compounds and CO which can simultaneously and efficiently treat fluorine-containing compounds and CO and are low in the running cost and can be simply controlled.
In order to solve the above described problems, the present inventors have made strenuous investigations, and found that the above described object can be achieved by first reacting a gas containing fluorine-containing compounds and CO with O2 and H2O without using a CO treating catalyst at a specified temperature or higher to oxidize the CO to CO2, and then contacting the gas with xcex3-alumina as the catalyst to decompose the fluorine-containing compounds, in treating the gas containing the fluorine-containing compounds and CO.
Namely, according to the present invention, there is provided a process for treating a gas containing fluorine-containing compounds and CO which comprises contacting the above described gas with O2 and H2O at a temperature of 850xc2x0 C. or higher to oxidize the CO to CO2; and subsequently contacting the gas with xcex3-alumina heated at 600 to 900xc2x0 C. to decompose the fluorine-containing compounds.